Pure red cell aplasia (PRCA) sometimes accompanies thymoma. Herein, we report a PRCA patient with thymoma with a clonal disorder of T cells. A 55-year-old man presented with anemia and anterior mediastinum tumor. The laboratory study revealed hemoglobin 8.2 g/dl; leukocytes 15.8 ร 10 9 /L with 76.5% neutrophils, 20.0% lymphocytes, and reticulocytes 0.0%. Bone marrow aspirate smears and biopsy sections revealed normal myeloid and megakaryocyte differentiation and contained no erythroid precursors. We made the diagnosis of PRCA. The size of the lymphocytes was small without any granules in the cytoplasm. The surface marker of peripheral blood mononuclear cells demonstrated increased CD2 + , CD3 + , CD4 -, and CD8 + populations. The mediastinal tumor was resected and a thymoma diagnosed. A monoclonal rearrangement of T-cell receptor (TCR)-โค-chain gene was found using Southern blot analysis of the mononuclear cells in both peripheral blood and thymoma. Treatment with prednisolone, thymectomy, and cyclophosphamide exerted no beneficial effect. After initiation of the Cyclosporin A therapy, the patient developed reticulocytosis. This PRCA case seems to present a neoplastic proliferation of CD8 + T cells in peripheral blood and thymus with a monoclonal rearrangement of the TCR-โค-chain gene. Am.